1. Field of the Invention
The invention relates to a method of cleaning crude gas, particularly waste gas from a mineral fibre production plant and contaminated with phenol and/or fomaldehyde and/or the condensation products thereof, and wherein the crude gas is sprayed with a circulated washing liquid enriched with micro-organisms, some of the impurities contained in the crude gas being in this way agglutinated, the washing liquid being collected and the impurities contained therein being at least partly degraded by the micro-organisms, the micro-organism being themselves subjected to an activation process.
Furthermore, the invention relates to an apparatus for carrying out the method, comprising a filter plant and a storage tank for the micro-organism enriched washing liquid, the storage tank being connected by a pipe and pump to the filter plant which discharges into the storage tank.
2. Background of the Prior Art
In the case of many production plants, e.g. in foundries or in plants which are of particular interest and which are for producing glass and mineral fibres as well as felts, fleeces or the like from these fibres, contaminated crude gas arises which must be cleaned before it can be passed out into the atmosphere.
The crude gas is first and foremost contaminated by phenol both in free and also in broken down form and formaldehyde together with phenol formaldehyde resin, which substances originate from the binding agents used in the production of the mineral fibre products. Furthermore, the crude gas contains odorous substances which smell unpleasant to humans and unpleasant and changing colourings.
The way in which emissions of such a production plant can be ascertained and restricted can be determined from VDI Guidelines 3457.
The limit emission values according to these Guidelines amount for water vapour volatile phenols and formaldehyde to 20 mg/cu.multidot.m in the case of annealing purposes and cooling apparatus and 40 mg/cu.multidot.m in the case of fibre manufacture.
A known method of cleaning crude gas resides in passing the gas through a washing and deposition chamber and subsequently through a wet precipitator, the washing water being recycled. When the crude gas is sprayed with washing water, aerosols form in which a major part of the organic impurities is absorbed and separated off in the precipitator. Certainly, the washing water cannot be used unrestrictedly and must be renewed from time to time, so giving rise to the problem of washing water cleaning. A further disadvantage lies in the fact that gaseous emissions can only be inadequately separated off in the wet film precipitator; odorous substances and colouring matter continue to pass into the atmosphere. Furthermore, phenol formaldehyde resin residues stock to the plates of the wet precipitator.
On the other hand, what are known as biowashers have already been proposed for crude gas cleaning, wherein some components of the waste gas are picked up by a washing liquid which is then regenerated by micro-organisms which use the washed out gas components as nutrients. The micro-organisms together with the undissolved pollutants form what is referred to as activated sludge. Since not all microorganisms can degrade certain pollutants, a natural selection takes place in the washing process. As a rule, the activated sludge must be ventilated to feed it with oxygen; furthermore, by virtue of evaporation, a fresh water topping up arrangement is required (VDI Guidelines 3478).
However, a biowasher has the disadvantage that aerosols such as bonded phenol, are poorly separated.